Field of the Invention
The present invention relates to image capturing apparatuses and control methods thereof, and particularly relates to an image capturing apparatus that converts an analog signal read out from an image sensor into a digital signal and to a control method thereof.
Description of the Related Art
Conventionally, a television standard called “full high-definition”, in which there are 1,920 horizontal pixels and 1,080 vertical pixels, has been commonly used. However, recent years have seen a continuing transition to a television standard called “4K2K”, in which there are 3,840 horizontal pixels and 2,160 vertical pixels. Further transitions to a next-generation television standard called “8K4K” (“ultra-high-definition”), in which there are 7,680 horizontal pixels and 4,320 vertical pixels, are expected in the future. As the number of pixels increases, framerates continue to increase as well.
The transition to such television standards has resulted in increased demand for more pixels and higher framerates in image capturing apparatuses that shoot video for television, and increasing the speed at which an image sensor reads out such video is an issue with respect to meeting such demand. To increase the readout speed, it is absolutely necessary to increase the processing speed of an AD converter provided in the image sensor, and a variety of techniques have been proposed with respect to increasing the speed of an AD converter.
In addition to increasing the readout speed, there is also high demand for an improvement in tone accuracy with the aim of improving S/N ratios and expanding dynamic ranges. Thus improving the tone accuracy without increasing circuit scales, lengthening processing times, and so on is an important issue with regards to future improvements in image quality.
Japanese Patent Laid-Open No. 2012-080252 discloses an image capturing apparatus having the following configuration for responding to such an issue. A first pixel signal obtained by amplifying a pixel signal at a first gain and a second pixel signal obtained by amplifying the pixel signal at a second gain that is greater than the first gain using a column amp circuit within the image sensor are AD (analog-digital) converted using different AD converter circuits. Then, one of the AD-converted first pixel signal and second pixel signal is selectively output in accordance with the level of the pixel signal. Employing such a configuration makes it possible to expand the dynamic range and improve the S/N ratio.
Japanese Patent Laid-Open No. 2012-080252 furthermore proposes a technique that level-shifts the selectively read-out first pixel signal and second pixel signal to the same gain level, detects gain error or offset error, and corrects the pixel signal on the basis of the detected value. By carrying out such processing, signal level gaps produced when combining the selectively read-out first pixel signal and second pixel signal into a single image can be reduced.
However, Japanese Patent Laid-Open No. 2012-080252 does not take into consideration variations (fluctuation) in correction values calculated using the detected values and variations (fluctuations) over time in the detected values for calculating gain error and offset error. Sudden changes in temperature, switching methods of driving the image sensor, noise in the power source supplied to column amps, noise in interconnects, external noise (magnetic noise produced during motor driving jumping in, for example), and so on are the main causes of fluctuations in correction values and detected values. Signal level gaps will be visible if the first pixel signal and the second pixel signal are combined into a single image as disclosed in Japanese Patent Laid-Open No. 2012-080252 with the correction values having fluctuated due to such causes. Signal level gaps will also be visible if the gain error and offset error are calculated with the detected values having fluctuated and the pixel signals of effective pixels are corrected using those calculated values, and the first pixel signal and the second pixel signal are combined into a single image as disclosed in Japanese Patent Laid-Open No. 2012-080252.
For example, in the case of a correction method in which a correction value is calculated on a frame-by-frame basis and updated, a visually unnatural video will be produced if frames having signal level gaps and frames not having signal level gaps are both present.
Meanwhile, in the case of a method that calculates an individual correction value for each of regions obtained by dividing an image of a single frame and corrects using those correction values, regions having gaps and regions not having gaps will be present from region to region, again resulting in a visually unnatural image. Likewise, in the case where an individual detected value is obtained for each of regions obtained by dividing an image of a single frame and each region is then corrected using a correction value calculated from that detected value, regions having gaps and regions not having gaps will be present from region to region, resulting in a visually unnatural image. This is particularly noticeable in video in which the subject contrast is low and the brightness level changes gradually.